The present invention relates generally to waste heat recovery and use of recovered waste heat from a fluid such as the exhaust of a vehicle engine.
When operating an internal combustion engine, a significant portion of the energy from the combustion is wasted as heat in the exhaust gasses expelled to atmosphere. Some have proposed systems for recovering exhaust gas waste heat in vehicles, such as transferring exhaust gas heat to engine coolant. However, there may be certain operating conditions where this is not desirable. For example, heat transfer may be undesirable if additional heat transfer may cause the liquid to boil, in which case the powertrain cooling system would need to have a higher capacity (larger radiator and fan) to prevent this.
Accordingly, some have suggested vehicle systems to allow for selectively transferring heat from the exhaust gasses to the engine coolant. For example, some vehicle exhaust systems include bypass pipes that run in parallel with a portion of the main exhaust gas stream, with flow control valves in the exhaust system to direct the flow. The flow control valves selectively redirect exhaust gas from main gas stream, through the separate bypass pipes that run in parallel with the main gas stream, and back into the main exhaust stream after passing through a heat exchanger. The heat exchanger in the bypass pipes transfers heat from the exhaust gasses to engine coolant that is directed through the heat exchanger. In this way, heat can be transferred to the engine coolant when desired. When heat transfer from the exhaust is not desired, the valves are actuated to direct the exhaust gas straight through exhaust system, bypassing the bypass pipes and heat exchanger. But such systems require additional package space for bypass pipes and heat exchangers as well as valves (and actuators for the valves) for redirecting the flow. Moreover, by diverting the exhaust gasses, there may be undesirable pressure drop variations in the exhaust system or undesirable changes in exhaust back pressure when switching from one flow path to another.